The present invention is directed to a method of blast cleaning electronic hardware components. More particularly, the method of the present invention is directed to cleaning electronic hardware components by blast cleaning with a water soluble abrasive media under mild conditions.
Electronic hardware components include any conductive, semiconductive or dielectric part used in electronic apparatus such as computers and the like. Such hardware components include, but are not limited to, electronic components of circuit and wiring boards, semiconductor wafers from which semiconductor chips are made, and disk drive heads and the like.
The cleanliness of electronic hardware components is regarded as being critical to their functional reliability. Contaminants on the electronic hardware components can cause shorts in the electronic hardware components and interfere with the operation of the electronic components such that the electrical functions of an electronic apparatus in which they are employed become unreliable.
In the manufacture of electronic hardware components, contaminants can accumulate after one or more steps of the manufacturing process as well as during the assembly of the components into an electronic apparatus. During the manufacturing process, electronic hardware components can be plated, etched, handled by operators during assembly steps, coated with corrosive or potentially corrosive fluxes and the like. Thus it is important that such contaminants be removed from the components. However, certain methods used to clean electronic hardware components can present undesirable problems of their own.
For example, in the fabrication of electronic circuit assemblies, e.g., printed circuit boards, soldering fluxes are first applied to the substrate board material to ensure firm, uniform bonding of the solder. These soldering fluxes fall into two broad categories: rosin and non-rosin, or water soluble, fluxes. The rosin fluxes which are generally only moderately corrosive and have a much longer history of use are still widely used throughout the electronics industry. The water soluble fluxes which are a more recent development are being used increasingly in consumer products applications. Because water soluble fluxes contain strong acids and/or amine hydrohalides, such fluxes are very corrosive. Unfortunately, residues of any flux can cause circuit failure if residual traces of the material are not carefully removed following soldering and thus remain on an electronic circuit assembly.
While water soluble fluxes can be easily removed with warm, soapy water, the removal of rosin flux from printed circuit boards is more difficult and has therefore traditionally been carried out with the use of chlorinated hydrocarbon solvents such as 1,1,1-trichloroethane, trichloroethylene, trichloromonofluoromethane, methylene chloride, trichlorotrifluoroethane (CFC113), tetrachlorodifluoroethane (CFC112) or mixtures or azeotropes of these and/or other solvents. These solvents are undesirable, however, because they are toxic and when released into the environment deplete the ozone layer and/or contribute to the greenhouse global warming effect. Thus, use of such solvents is subject to close scrutiny by the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA), and stringent contaminant equipment must be used. Moreover, if released into the environment these solvents are not readily biodegradable and are thus hazardous for long periods of time.
Alkaline cleaning compounds known as the alkanolamines, usually in the form of monoethanolamine, have been used for rosin flux removal as an alternative to the toxic chlorinated hydrocarbon solvents. These high pH compounds (e.g., about pH 12), chemically react with rosin flux to form a rosin soap through the process of saponification. Other organic substances such as surfactants or alcohol derivatives can be added to these alkaline cleaning compounds to facilitate the removal of such rosin soap. Unfortunately, these compounds, as well as the water soluble soldering fluxes, have a tendency to cause corrosion on the surfaces and interfaces of printed wiring boards if such compounds and fluxes are not completely and rapidly removed during the fabrication process.
The complete removal of adhesive and other residues also pose a problem. During the manufacture of electronic circuit assemblies the components are mounted on the upper surface of the board with leads protruding downwardly through holes in the board and are secured to the bottom surface of the board by means of an adhesive. Further, it is sometimes necessary to temporarily protect certain portions of the circuit board by the application of special adhesive tape to susceptible areas. Once such protection is no longer needed, the adhesive tape must be removed. In both instances, a residue of adhesive generally remains which, if not thoroughly removed, can cause premature board failure. Removal of this adhesive residue has traditionally been carried out by the use of chlorinated solvents which, as already described, are toxic and environmentally undesirable.
In addition to soldering fluxes, any photoresist used in etching and screen printing of components on circuit and wiring boards can also cause short circuiting problems. U.S. Pat. No. 5,145,717 to Drury and assigned to E. I. Du Pont de Nemours and Company discloses a method of blast cleaning printed circuit boards of photoresist with an abrasive. The method employs polymeric or resin material having a Mohs scale hardness of about 2 to 4. The particles are accelerated at the surface of the board at an angle of about 15 to 90 degrees by means of a blasting apparatus to remove the photoresist. Although the method may remove photoresist from circuit and wiring boards, the polymer particles used to remove the photoresist can not be easily removed from the blasted surface. Moreover, the polymer particles can become embedded in the blast cleaned surface, or lodged in dead areas,i.e., corners or recesses of the surface, where it is very difficult to remove the polymer particles. Also, the photoresist can not be readily disposed of by sewering. Such particles would pollute the environment.
Similar contaminant problems also occur with disk drive heads. During assembly of computer terminals, such contaminants as adhesive and finger prints can contaminate the drive heads and result in compromised performance of the electronic apparatus.
As with the electronic components of circuit and wiring boards, and disk drive heads, it is important that semiconductor wafers and chips are as clean as possible before the semiconductor material is incorporated into any electronic apparatus. Any contaminants on the semiconductor material can jeopardize the optimum performance of the electronic apparatus. Consequently, such contaminants as oxidation compounds and laser slag formed on semiconductor wafers during manufacture preferably are removed prior to dicing or cutting the semiconductor wafers into semiconductor chips.
Blast cleaning of semiconductor wafer surfaces has been previously used to wash electrostatic particles from the surface. One such method utilizing water is disclosed in U.S. Pat. No. 4,027,686 to Shortes et al. and assigned to Texas Instruments, Inc. Other fluids, including air, have been used with varying degrees of success. Although various methods have been employed to clean contaminants from electronic hardware components, there is still a need for a mild and environmentally friendly method to clean such contaminants as photoresists, solder flux, adhesives, oxidized material, laser slag and the like from electronic hardware components.
It is known to blast clean soft surfaces of aluminum, magnesium, and plastic surfaces to remove paint and dirt with such abrasives as sodium chloride and sodium bicarbonate. However, in order to maintain the flow of such abrasives as sodium bicarbonate, flow aids such as hydrophobic silica are often employed. Such flow aids, however, can leave additional deposits on the surface of electronic components being cleaned. Such deposits when formed on electronic hardware components are not only difficult to remove even with high pressure water rinses, but replace one type of contaminant with another.
A primary object of the present invention is to provide a method of blast cleaning contaminants from electronic hardware components by blast cleaning the contaminants from the surface of electronic components under mild conditions.
Another object of the present invention is to blast clean the surface of electronic hardware components using relatively low pressures.
A further object of the present invention is to provide a method of blast cleaning contaminants from electronic components with environmentally friendly abrasives.
Still yet another object of the present invention is to blast clean electronic hardware components with a water soluble abrasive media.
Other objects and advantages of the present invention will become apparent to those of skill in the art upon reading the description of the present invention which follows and practicing the present invention.